The invention relates to a process for the preparation of shaped translucent lithium disilicate glass ceramic products which can be prepared as blanks, which may be processed to shaped translucent dental products with high strength, particularly by plastic shaping with the action of pressure and heat or by machining.
Lithium disilicate glass ceramics are known from the prior art. Thus, self-glazed lithium disilicate glass ceramic articles are described in EP-B-536 479 but are not intended for dental purposes. The glass ceramics also contain no La2O3, and there is likewise no description of the preparation of blanks from the glass ceramic which, after processing, undergo a further heat treatment in order to complete crystallization. It is also necessary to carry out the heat treatment at a very low rate of heating of 5K/min in order to prevent stresses in the structure of the glass ceramic. Moreover, the glass ceramic is intended primarily for the preparation of tableware which naturally has only low translucence.
EP-B-536 572 also describes lithium disilicate glass ceramics which contain no La2O3. By scattering a finely divided coloured glass onto their surface, they receive structure and colour, and are used as lining elements for building purposes.
Lithium disilicate glass ceramics are disclosed in U.S. Pat. No. 4,189,325 which necessarily contain calcium oxide for improving the flow and also platinum and niobium oxide as special nucleating agents in order to produce very fine and uniform crystals. Even though the glass ceramic can be prepared in the form of blanks which have not yet crystallised completely, it is nevertheless free from La2O3.
WO-A-95/32678 and U.S. Pat. No. 5,507,981 describe lithium disilicate glass ceramics which may be processed to shaped dental products by hot pressing using a special pressable crucible. The glass ceramic materials are heated to such an extent, however, that crystals are no longer present in the molten material, otherwise the viscosity is too high for pressing to the dental product. Tests have shown that when the materials described are pressed by means of the process described in EP-A-231 773 and using the pressing furnace disclosed therein, an undesirably strong reaction occurs with the investment material used. Moreover, the glasses used show a very high rate of crystal growth, so that large crystals are produced during the heat treatment which impair the structure of the glass ceramic produced and consequently lead to products with poor strength.
Moreover, glass ceramics based on SiO2 and Li2O are known from DE-C-1 421 886 which contain large quantities of arsenic trioxide which is physiologically very harmful.
A lithium disilicate glass ceramic which is suitable for the preparation of dental crowns and bridges but contains no La2O3 at all is disclosed in U.S. Pat. No. 4,515,634.
The glass ceramics described in FR-A-2 655 264 are free from La2O3. They contain lithium oxide and silicon oxide and very large quantities of MgO and are suitable for the preparation of dental prostheses.
Blanks of sintered ceramic based on leucite, feldspar or mica which are processed to dental products by computer-aided milling processes are also known from the prior art. These products have low strength, however, which is why said materials have not become established for highly stressed dental restorations.
The known lithium disilicate glass ceramics exhibit shortcomings when they are further processed to shaped products since an undesirably strong reaction with the investment material used during pressing occurs when they are processed in the plastic state using elevated temperatures and elevated pressures. Further processing of the glass ceramics by machining, such as milling, cannot usually be carried out satisfactorily due to the strength and toughness of the glass ceramics. Moreover, the conventional lithium disilicate glass ceramics frequently do not exhibit the high strengths and optical properties such as high translucence required for dental products and in many cases they also lack the chemical stability required for use as dental material which is permanently flushed with fluids of various kinds in the oral cavity.
The object of the invention is, therefore, to provide a process for the preparation of shaped translucent lithium disilicate glass ceramic products which have good chemical stability, a low density of defects, and high translucence with simultaneously good mechanical properties and exhibit only little reaction with the investment material used when further processed by pressing in the plastic state, and the glass ceramic products may also be prepared in the form of blanks with a low degree of crystallisation which may be shaped easily in the desired manner by mechanical means such as machining and may be converted to a high-strength glass ceramic product by a subsequent heat treatment.
Said object is achieved by the process for the preparation of shaped translucent lithium disilicate glass ceramic products.
The invention also relates to the shaped glass ceramic products, the use of such products and the shaped dental products.
The process according to the invention for the preparation of shaped translucent lithium disilicate glass ceramic products is characterised in that
(a) a melt of a starting glass is produced which contains the following components:
(b) the melt of the starting glass is shaped in the desired manner and cooled, and
(c) the shaped glass product is subjected to at least one heat treatment in the temperature range from 400 to 1100xc2x0 C. in order to obtain a shaped glass ceramic product in the form of a blank.
In process stage (a), a melt of a starting glass is produced, to which end suitable starting materials, such as carbonates, oxides, phosphates and fluorides, are intimately mixed and heated to temperatures of, in particular, 1200 to 1600xc2x0 C. In order to obtain a particularly high degree of homogeneity, the glass melt obtained may be poured into water to form glass granules and the glass granules obtained are melted again at temperatures of, in particular, 1200 to 1600xc2x0 C. for 1 to 4 hours.
The melt of the starting glass preferably contains at least one of the following further components:
Surprisingly, it was established that the additional incorporation of ZrO2 led to an increase in translucence, although the opposite effect was observed in the conventional glass ceramic according to EP-B-536 479.
Ranges that may be chosen independently of one another, unless otherwise specified, exist for the quantities of the individual components, said ranges being as follows:
For example oxides of f-elements may be used as colour components or fluorescent components. In preference, at least one of the following compounds is used.
The special oxides that can be used as colour or fluorescent components in the process according to the invention ensure that the colour of the glass ceramic product can be matched easily to the application in question. This is particularly important if the glass ceramic products are to be used as dental products, the colour of which must be matched specially to that of the natural tooth material of the patient in question. The colour spectrum that can be obtained with these special oxides ranges from very pale shades to deep grey-brown shades e.g. in the case of non-vital tooth stumps. The fluorescence of the natural tooth material is imitated by any fluorescent components present. A particular advantage of the colour and fluorescent components used according to the invention is that they do not interfere with the structure of the glass ceramic products produced in such a manner that non-homogeneous materials with a high density of defects and high porosity are produced. This problem frequently occurs with sintered ceramics, the colour of which is altered by the addition of pigments. In order to prevent any deterioration in their colouring effect, said pigments are not usually added until prior to the sintering process carried out at relatively low temperatures so that they are always present as crystals or crystallites which lead to non-homogeneities.
Apart from the components mentioned above, the starting glass may also contain additional components for which in particular B2O3, Na2O, BaO, F and/or SrO are suitable.
Preferably, the melt of the starting glass is composed of the components mentioned in the stated quantities.
Moreover, the melt of the starting glass is shaped in the desired way in stage (b) and cooled. Shaping takes place in particular by pouring the melt into a desired mould. It is also possible for compaction of the melt by pressure to take place after pouring in order to achieve a particularly good homogeneity and accurate reproduction. It is possible to proceed in such a manner that a glass droplet is introduced into the desired mould and then compacted by pressing.
The melt is cooled particularly in a controlled manner so as to prevent stresses in the structure associated with rapid temperature changes and to prevent cracks and fissures that may possibly result from said stresses. As a rule, the melt is therefore poured into preheated moulds or cooled slowly in a furnace.
Finally, the shaped glass product formed undergoes at least one heat treatment in stage (c) in order to bring about the crystallisation thereof. When this process stage has ended, a shaped glass ceramic product in the form of a blank is obtained. This blank usually takes the form of a small cylinder or a rectangular block. The heat treatment takes place preferably at a temperature of less than 1000 and particularly less than 900xc2x0 C. The shaped glass product is preferably introduced into a furnace already heated to the temperature mentioned. In contrast to conventional materials, it is not necessary to select a slow rate of heating in order to prevent stresses. The special composition and method of preparation of the material according to the invention is apparently responsible for this advantageous behaviour.
The degree of crystallisation and the crystal size in this glass ceramic blank may be varied very widely by the type of heat treatment selected. On the one hand it is possible to produce a glass with only nuclei or very small crystals in the sub-micron region, which thus represents the simplest form of a glass ceramic, or on the other hand to form a fully crystallised glass ceramic. In each case, the ceramic production process takes place by way of the mechanism of volume crystallisation, and volume nucleating agents such as e.g. P2O5 present in the starting glass used play an important part in the formation of finely divided crystals in the structure.
In particular the following two possibilities (d1) and (d2) are available for producing the final glass ceramic product, such as a dental bridge or a dental crown.
On the one hand, the glass ceramic product in the form of a blank may undergo plastic shaping in stage (d1) to a glass ceramic product of the desired geometry at a temperature of 700 to 1200xc2x0 C. and by the application of pressure, particularly of 8 to 40 bar. It is preferable for this forming stage to use the process described in EP-A-231 773 for the production of dental restorations and to use the pressing furnace likewise disclosed therein. In said process, the blank is pressed in the plastic state into a mould cavity corresponding to the desired shaped dental product, such as crowns, using heat and pressure. The pressing furnace used in particular for this purpose is sold as the Empress (copyright) furnace by Ivoclar AG, Liechtenstein.
It has become apparent that conventional lithium disilicate glass ceramics exhibit an unacceptably strong reaction with the investment material used during further processing to glass ceramic products, have insufficient flow properties or exhibit uncontrolled crystal growth. These disadvantages are avoided in the process according to the invention by the use of La2O3 and optionally Al2O3 in the stated quantities in the starting glass. As a result, the glass ceramic product in the form of a blank may be processed in an advantageous manner by pressing in the plastic state to a glass ceramic product of the desired geometry, particularly a dental product such as a dental restoration.
It is also possible to process the glass ceramic product in the form of a blank by machining in stage (d2), particularly by CAD/CAM-based milling devices, to obtain a glass ceramic product of the desired geometry. A so-called chair-side treatment is thus possible for the dentist. When this variant of further processing is carried out, the glass ceramic blank used in particular is one which is not yet fully crystallised but is present e.g. only as a nucleus-containing glass blank or glass ceramic blank with very small crystals. Such glass ceramic blanks that have not yet fully crystallised have the particular advantage that they may be machined to the finished glass ceramic product of the desired geometry in a markedly easier manner than conventional glass ceramics. In order to produce a glass ceramic blank in which the glass matrix contains only nuclei or very small crystallites, it has proved to be particularly advantageous to carry out the heat treatment performed in stage (c) at a temperature of 400 to 900xc2x0 C. In each case, the degree of crystallinity of the glass ceramic blank used may be adapted to the type of machining desired so that said machining may be carried out as easily as possible.
After the subsequent machining in stage (d2), the shaped glass ceramic product obtained then undergoes at least one further heat treatment, particularly at 700 to 900xc2x0 C., in order to achieve further crystallisation and hence solidification of the glass ceramic product. The fracture strength, colour and translucence are improved by this further heat treatment.
The finished glass ceramic product of the desired geometry present after further processing, particularly in stages (d1) and (d2), may ultimately be provided with a coating, which is advantageous if it is used in the dental field. Suitable coatings are in particular a ceramic, a sintered ceramic, a glass ceramic, preferably an apatite glass ceramic, a glass, a glaze and/or a composite. Those coatings that have a sintering temperature of 650 to 950xc2x0 C. and a linear thermal expansion coefficient that is smaller than that of the glass ceramic product to be coated are advantageous. Coatings whose thermal expansion coefficient deviates by not more than xc2x13.0xc3x9710xe2x88x926 Kxe2x88x921 from that of the substrate are particularly suitable.
A coating is applied in particular by sintering on. During this sintering process, the glass ceramic product containing the lithium disilicate glass ceramic is, however, brought to a temperature range which lies above the transformation point of the residual glass matrix of the glass ceramic. In so doing, conventional lithium disilicate glass ceramics are frequently deformed in an unwanted manner because their dimensional stability on heating is too low. The glass ceramic product prepared according to the invention, however, shows excellent dimensional stability on heating, for which in particular the La2O3 content and possibly the Al2O3 content in the stated quantities is responsible.
The glass ceramic products prepared according to the invention are particularly suitable for use as dental products or constituents thereof due to their properties. Preferred glass ceramic products have a 3-point bending strength of more than 400 MPa if they are prepared according to process variant (d1) and of more than 250 MPa if they are prepared according to process variant (d2). The process used to determine the 3-point bending strength is explained in the Examples.
Moreover, the glass ceramic products according to the invention have a translucence comparable with that of the natural tooth. In order to quantify the translucence, the CR value was determined according to the method described in the Examples. The CR value, also known as the contrast ratio, indicates the ratio of light reflection of a specimen of the glass ceramic on a black background to the measurement of light reflection of a specimen on a white background and thus serves as a measure of the translucence of a material. The CR value is defined by the following formula:
CR=Yb/Yw 
where
CR=contrast ratio
Yb=light reflection of the specimen on a black background, and
Yw=light reflection of the specimen on a white background.
The CR value is always between 0 and 1, whith CR=0 standing for an opacity of 0% and consequently a completely translucent material, and CR=1 standing for an opacity of 100% and consequently a completely opaque material, i.e. one which is impervious to light.
The glass ceramic product according to the invention usually has a CR value of 0.05 to 0.9 and preferably 0.1 to 0.75, in each case measured with a sample thickness of 1.2 mm.
Analyses of the glass ceramic product according to the invention have also shown that this has a very homogeneous structure with uniformly distributed fine crystals. It is assumed that this special structure is brought about by the particular composition of the starting glass used and by the shaping, particularly pouring solid glass blanks in stage (b), and is responsible for the particularly high strength of the glass ceramic product eventually obtained.
It is also surprising that the colour, translucence and fluorescence of the glass ceramic product according to the invention may be matched to that of a natural tooth without the colour and fluorescent components used adversely affecting the strength and toughness of the glass ceramic. In contrast, it is known that with glass ceramics based on leucite, crystallisation is considerably affected by such additives and the strength is often very much reduced. It is known that the pigments used in many cases in sintered ceramics lead to a very high density of defects and to pore formation in the glass ceramic, which in turn impairs the properties thereof.
Finally, the glass ceramic product according to the invention is characterised by excellent acid resistance, which is preferably less than 100 xcexcg/cm2 loss of mass. Said loss of mass was determined by the method explained in the Examples in which the glass ceramic is treated with aqueous acetic acid over a certain period and the loss of mass ascertained after the treatment serves as a measure of acid resistance.
Preferred shaped dental products which contain the glass ceramic product according to the invention are dental restorations such as an inlay, an onlay, a bridge, an abutment, a facing, a veneer, a facet, a crown or a partial crown.
Moreover, preferred shaped dental products are those in the form of blanks or ingots, i.e. which undergo further processing to the final dental product, e.g. according to stages (d1) and (d2). Such blanks may be present in various forms adapted to the further processing method in question, such as small cylinders or rectangular blocks.
The invention is explained in more detail below on the basis of Examples.